1. Field of the Invention
The present invention relates to radar techniques. More particularly, it concerns radars set up on the ground and used for the detection and tracking of moving targets of any kind such as, for example, aircraft in flight, especially for airport approach control.
The invention can be applied to any radar system for the identifying of moving targets in which moving targets, that are useful in principle, and fixed targets, in principle not useful, are differentiated by using the variations in amplitude and phase of the received signal on several successive pulse recurrences of the radar. In radars such as this, the distinction between moving targets and fixed targets in a fixed target canceller at output of the receiver of the radar is limited by the variations in amplitude and phase of the signal caused by the radar itself, both in the transmission circuits and in the reception circuits of the radar.
The variations in amplitude and phase of the received signal have been used, since 1950 at least, to distinguish moving targets from fixed targets. A useful reference on this subject can be found in Merril I. SKOLNIK, Radar Handbook, McGRAW-HILL, and especially in chapter 17 thereof, by William W. SHRADER, pages 17-2 to 17-50. Very early on, the specialists discovered that a technique such as this imposes a degree of stability in amplitude and phase, on the transmitter/receiver set of the radar, that is compatible with the overall performances sought. This stability in transmission and reception has therefore quickly become one of the major technical characteristics of the transmitter/receiver set of radars of this type, called radars that are "coherent" or "made coherent", depending on their structure, and the specialists have therefore striven to specify and measure it.
Appropriate methods of measurement have been found and special instruments have been built to perform this type of measurement.
The method most commonly used at present consists in memorizing, in digital form, two video voltages I and Q of the receiver in the presence of a fixed target. One of the voltages, I, is called a phase video voltage and the other voltage, Q, is called a video voltage in quadrature. The fixed target is either real or simulated by the injection, into the receiver, of a delayed or non-delayed sample of the transmitted signal (cf. FIG. 1). As the radar transmits with the specified frequency of renewal, a sequence of pairs (I, Q) is thus formed. Each pair (I, Q) is converted into two words of digitized samples, each having a number of bits corresponding to the analog-digital converters used, which are generally those included in the canceller of the radar itself. The variations observed in the successive samples represent the instability measured and are analyzed either in the temporal field, according to a computation of means and standard deviations, or in the frequency field according to a computation by Fourier transform and analysis of the spectrum obtained.
The computation by Fourier transform is generally preferred because its results are direct and convenient to interpret in the increasingly common case where modern radars process the signals received by a bank of Doppler filters.
The measurement of the stability of a radar in transmission/reception is a difficult operation since it requires the measurement of small variations in a voltage signal having a high amplitude. The instruments used for the measurement have been improved in recent years to make them compatible with the increased performance characteristics of the radars as regards stability.
At present, the stability levels in demand are such that the measuring instruments used up to now do not give the quality and precision required to make the measurement with a sufficient margin between the value of the parameter to be measured and the faults introduced by the measuring instrument itself.
In particular, the two analog-digital converters used in most of the radars and receiving the two video voltage signals I and Q have an encoding dynamic range that is limited by the number of significant bits at output. This number of bits is itself limited by considerations of cost and technical and technological feasibility.
The number of significant bits of the analog-digital converters commonly used is, at present, equal to 12 bits. This corresponds to 2.sup.12 =4096 voltage levels between limit voltages -V.sub.MAX and +V.sub.MAX on each of the two voltage paths I and Q. It is assumed that, to encode the noise and other weak signals properly, these signals should occupy at least three or four encoding levels on either side of zero. The result thereof is a measuring dynamic range of the order of 4096 divided by 2.times.3=6 or 2.times.4=8 giving, in decibels: 20 log (4096/(6 or 8))=54 or 57 dB.
This dynamic range is insufficient to measure the stabilities of the radar system of the order of -60 dB or more, and even more insufficient to measure the partial contributions resulting therefrom, which are of the order of -65 to -70 dB.
A known approach to this problem of measurement consists in making use of analog/digital converters having a greater number of bits. This rise in the number of bits increases the dynamic range of measurement to the detriment of the cost of the measuring system, especially when an example of the measuring system has to be given with each radar delivered, or when the measuring system has to be integrated into the radar itself.